Scathach is only available from this banner. Medea (Lily) can be summoned from this banner even if you haven't completed Okeanos.

Scathach Trial Quest

The Scathach Trial Quest is available for the duration of the summoning event from Chaldea Gate. It is an easy quest that does not allow player support Servants, only the following NPC support Servants: Scáthach, Diarmuid Ua Duibhne, and Fergus mac Róich.